Overhead projectors of this kind are produced by different manufacturers in a variety of types. These overhead projectors have a foot in the form of a plate on which the original copy which is to be projected is laid. Such plate is the support for a reflecting Fresnel lens. If transparent originals are laid on the lens they are diascopically projected. However, with this projector, non-transparent originals can be episcopically projected. On the edge of the plate, in general on a corner of the plate, there is mounted a post which carries a housing which on the one hand accomodates the light source comprising a lamp, convex mirror and a condenser and at the same time accomodates the objective lens. There are embodiments in which the power supply is accommodated in a casing or underneath the plate serving as a foot. However, this is disadvantageous because the dimensions of the apparatus are unnecessarily large and because it is necessary to provide a ventilator for cooling the power supply and another ventilator for cooling the lamp. Because of this, there are embodiments in which the power supply is accommodated in the housing that accomodates the light source and the objective lens. However, this has also been found disadvantageous because the weight of the housing is thereby considerably increased and hence the post and possibly a bracket between the housing and the post must be made particularly strong. In many cases, this housing is given a further weight increase in that, adjacent the objective for overhead projection, still a further objective for slide projection together with guiding and shifting means for the slides are accommodated in the housing. The strong design of the post is disadvantageous not only because the price and weight of the overhead projector are increased, but also because of the need of a very strong fastening of the post on the foot plate. This fastening must be made so fast because considerable moments produced through the weight of the housing, the arrangement of this weight laterally of the post and the considerable spacing of this weight from the fastening point act on the fastening. In general, the post does not have the desired rigid fastening but is prone to swing which allows the projection to be unclear and unsharp.
The requirement of a very rigid and strong fastening at the junction of the post to the housing has lead heretofore to making this fastening only rigid.
There are indeed also hinged fastenings of the post on the housing but only with overhead projectors in which the light source is underneath the writing plate. Such disdiascopically operating overhead projectors with hinged posts are used as portable apparatus, especially for taking on trips. Here the expense is driven higher if the post is to be free of swinging.
Also in the use of a projector on trips there is the problem that supply voltages are different in different countries. While 220 volt alternating current is standard throughout German, North America raises problems with 110 volt alternating current. There are also European countries with 110 and 130 volts and in other parts of the world direct current is used. As the usual commercial projectors are designed for a single voltage and for a single kind of current, it can happen to a lecturer that in another country, he cannot use the projector he has brought because there the electrical supply has a different kind of current or a different voltage.